With the increased use of wind energy for electricity generation, grid operators (TSO's or Transmission System Operators) have introduced stricter requirements for wind park operators with respect to active and reactive power generation in support of the grid. For example, wind park operators may be requested to not always generate maximum active power available depending on wind conditions. Also, in accordance with some grid codes, wind parks have to support the grid during a voltage drop and not e.g. disconnect from the grid.
In this respect, it is known to vary the reactive power generated in a wind park in accordance with a grid voltage at a Point of Common Coupling (PCC). It is also known to vary the active power generated in a wind park in accordance with a measured grid frequency.
EP 1 850 002 discloses a wind park comprising a number of fixed speed wind turbines and a number of variable speed wind turbines. The variable speed wind turbines are adapted to equalize an output of the wind park by compensating for a varying output of the fixed speed wind turbines. These variable speed wind turbines may be independent or a central management device may be provided for sending control commands to each of the variable speed wind turbines.
U.S. Pat. No. 7,531,911 discloses a method of reactive power regulation in a wind farm wherein a measured reactive power output of the wind farm is compared to a reactive power set point; based on the difference between them, a reactive power command is generated. The reactive power commands for each of the wind turbines are determined as a percentage of an instantaneous maximum reactive power capacity of each of the turbines.
US 2008/0073912 discloses a wind park including at least two wind turbines and a park master configured for power factor control and for sending global power coefficient signals to lower level regulators at the wind turbines.
In general, prior art control systems are known that are based either on a centralised control or on a decentralised control. An advantage of a centralised control approach may generally be that the control is stable. However a disadvantage of a centralised control may be that such a control is not always able to react sufficiently quickly to grid abnormalities. An advantage of a decentralised control may be that it is quicker to react to grid abnormalities, but a disadvantage can be that the control can be unstable.
There thus still exists a need for a power control in a wind farm that can combine stable control with quick reaction capability to grid abnormalities.
The present invention is aimed at partially or completely fulfilling this need. Further advantages will become apparent from the description that follows.